The present invention applies to a mass airflow measuring apparatus that measures the mass airflow through the suction air path of an internal combustion engine, particularly to an heating-resistor type mass airflow measuring apparatus that measures the mass airflow using an heating resistor.
In a conventional heating-resistor type mass airflow measuring apparatus, the current through the heating resistor is so controlled that the heating resistor temperature is higher by a specified temperature than that of the temperature-sensing resistor that detects the suction air temperature, and the suction mass airflow is measured in terms of the current through the heating resistor.
The conventional heating-resistor type mass airflow measuring apparatus is found to involve a problem that, when the engine has stopped and the airflow through the suction system has ceases, gaseous material flows by diffusion and evaporation out of the engine and its apparatuses installed in the suction system in the downstream of the heating-resistor type mass airflow measuring apparatus and reaches the installation point of the heating-resistor type mass airflow measuring apparatus and adheres onto the heating resistor, a mass airflow detecting element. In particular, since volatile gas such as oil vapor evaporates and diffuses when the temperature of the engine or its apparatuses installed in the suction system is higher than the vaporization temperature of the gas after the engine stoppage, and flows backward to the installation point of the heating-resistor type mass airflow measuring apparatus, it adheres onto the heating resistor exposed to the suction air. The adhesion of the gas results in the degradation of the measuring accuracy of the heating-resistor type mass airflow measuring apparatus because of corrosion and/or contamination of the heating resistor. In addition, the adhesion may result in accelerated adhesion of foreign substance such as dust onto the heating resistor. As a result, there arises a problem that the measuring accuracy of the mass airflow measuring apparatus degrades.
As an art relating to the present invention, there is disclosed a mass airflow measuring apparatus, for example, in a Japanese Application Patent Laid-Open Publication No. 3146850, where the apparatus is kept electrified continuously even after the engine stoppage so as to lessen the degradation of the measuring accuracy of the mass airflow measuring apparatus just after the engine start. This, however, continues electrification of the mass airflow measuring apparatus throughout the engine stoppage and the object of the electrification is to lessen the degradation of the measuring accuracy just after the engine start.
An object of the present invention is to offer a mass airflow measuring apparatus in which the effect of the adhesion onto the heating resistor caused while the engine is off is reduced and the measuring accuracy is improved.
(1) To achieve the above object, according to the present invention, there is provided an heating-resistor type mass airflow measuring apparatus, comprising an heating resistor which detects the mass airflow of the air sucked into an internal combustion engine and an electronic circuit which, connected to this heating resistor electrically, outputs a signal corresponding to the suction mass airflow, using the heating resistor; further comprising a temperature maintaining means which, after the internal combustion engine has stopped, maintains the heating resistor temperature equal to or higher than the temperature during the operation of the internal combustion engine until the temperature of the internal combustion engine and its apparatuses installed in the suction system has lowered below the generation temperature of volatile gas such as oil vapor.
With the above construction, it becomes possible to reduce the effect of the adhesion onto the heating resistor caused while the engine is off and improve the measuring accuracy.
(2) In the above (1), it is preferred that the mass airflow measuring apparatus further comprises a temperature detecting means which detects the temperature of the air sucked into the internal combustion engine, and the temperature maintaining means maintains the heating resistor temperature, based on the temperature detected by the temperature detecting means, equal to or higher than the temperature during the operation of the internal combustion engine even after the internal combustion engine has stopped.
(3) In the above (1), it is preferred that the mass airflow measuring apparatus further comprises a water temperature detecting means which detects the temperature of the cooling water of the internal combustion engine, and the temperature maintaining means maintaining the heating resistor temperature, based on the temperature detected by the water temperature detecting means, equal to or higher than the temperature during the operation of the internal combustion engine even after the internal combustion engine has stopped.
(4) In the above (1), it is preferred that the temperature maintaining means maintains the heating resistor temperature, based on the elapsed time after the stoppage of the internal combustion engine, equal to or higher than the temperature during the operation of the internal combustion engine even after the internal combustion engine has stopped.
(5) In the above (4), it is preferred that the temperature maintaining means maintains the heating resistor temperature, for the same length of time as the operation of the cooling fan of the internal combustion engine after the engine stoppage, equal to or higher than the temperature during the operation of the internal combustion engine even after the internal combustion engine has stopped.
(6) In the above (1), it is preferred that the temperature maintaining means judges the stoppage of the internal combustion engine based on the mass airflow detected by the heating resistor.
(7) In the above (1), it is preferred that the mass airflow measuring apparatus further comprises a means which adds a mass airflow dependency to the apparatus so that the heating temperature of the heating resistor becomes higher on the very-low mass airflow side.